I go about 2.5 to 3 inches with the spray. Depending on the dog. I extinct the water and food gradually, one or the other, but not both simultaneously. As with any training, extinct one variable at a time.
I walk directly on the spray, then plant my food drops on the way back, double laying the track. In winter, I try to use pavement that is free of salt from de-icing. Usually this is easy to do, by working on the higher elevation of a parking lot, as most are sloped to assist runoff. In the winter, I use a warm bottle, so the water doesn't immediately freeze to the pavement. Also bear in mind that you are shedding much less odor on the track in the cold, and with lots of clothing on. In summer, you can wear flip flops, bare feet, or wet socks, so you really put down a lot of odor.
To eliminate the visual factor, allow the water to eventually dry, and theoretically it should retain the scent. There are some out there that are skeptical of this system, but I have had great luck with multiple dogs, of various levels of drive, food drive, and motivation to track.
I have actually heard some people believing that HITT trained dogs will start seeking out water runoff in search of food or reward...this has not been my experience.
I like the system, especially for "real world" type tracks. I modify it a bit, and start the dog on grass scent pads, to encourage a deep, focused nose. As soon as the dog is solid, I'll run a few short "tails" off the scent pads. I then immediately transition to all hard surface, on pavement, then gradually introduce concrete, hard pack dirt, grass, then leaves and woods debris, in that order. (progressing from least porous/less scent retained to most) moving from most difficult surface the dog will ever track, to easiest. I like to do a lot of surface transitions early on, as that is what sometimes creates confusion for the dog, when going from one surface to another.
I have found it to be:
Easily taught, even to novice tracklayers.
Easy to keep the dog on tracks initially.
Definitely a way to track more easily during the winter, when you are buried in snow like we are right now.
That was quite a rant...so anything I can help you with, just let me know.